WIKA FLM - Handbuch (Seite 7)

GB
WIKA operating instructions level sensor model FFG 7
13445627.01 12/2010 GB/D/F
The level sensor shown in figure 2 serves for the continuous
measurement of fluids. To transmit the liquid level value to the sensor, a
float is sliding on the sensor tube. The operating principle of the sensor
is based on the magnetostriction effect. The float contains a magnet
while a wire made of magnetostrictive material is installed under tension
inside the sensor tube. The magnetic field of the float is twisting the
wire. A second, temporary magnetic field is generated along the wire by
a current pulse within the wire. The superposition of the two magnetic
fields will release a mechanical wave on the wire.
At the end of the wire within the sensor head, the mechanical wave is
converted by a piezo-ceramic transducer into an electrical signal. The
starting point of the mechanical wave and, hence, the float position is
then determined by measuring the transit time.
The results of such measurements are feasibility-tested by a microcon-
troller located in the sensor head and are then converted into a current
value by a digital analog transformer. The sensor is provided with a
2-wire connection so that the current consumption indicates the position
of the float at the same time. The current values are restricted to a range
of between 3.5 ... 23 mA.
2. Description of functions and design
Fig. 1: Level sensor system model FFG